Hydrogen storage alloys predominantly used these days in preparation of anodes for nickel-hydrogen rechargeable batteries are AB.sub.5 type alloys (CaCu.sub.5 type structure) that have light rare earth elements such as La, Ce, Pr, Nd, or a mixture of these elements (misch metal) in A-site, and Ni, Co, Mn, and/or Al in B-site. This kind of alloys has a larger hydrogen storage capacity than other alloys, and hydrogen absorption-desorption pressure (equilibrium pressure) of 1 to 5 atmospheres at an ordinary temperature, which make the alloys usable. These alloys are generally composed of 0.4 to 0.8 of Co, 0.3 to 0.5 of Mn, 0.2 to 0.4 of Al, and less than 3.9 of Ni per one rare earth metals in atomic ratio, for the sake of adjustability of equilibrium pressure and corrosion resistance against the electrolyte in the battery.
The nickel-hydrogen rechargeable batteries have recently made rapid prevalence in the field of laptop computers, cellular phones, and portable audio equipment. Electric vehicles equipped with such batteries will be put to practical use before long. As a result of diversification of battery usage, there have developed demands not only for high electrode capacity and long service life (cycle life) of the batteries, but also for high rate charge-discharge performance, i.e. heavy current charge-discharge performance or high rate discharge performance at lower temperatures, as important characteristics, to keep up with higher output of equipment or use in cold district.
For the purpose of improving the electrode capacity, JP-A-6-145851, for example, proposes to reduce the Ni content with respect to rare earth metal content, that is, an alloy of rare earth-rich composition. In this measure, however, the higher content of rare earth metals causes the corrosion resistance against electrolyte to decrease, resulting in disadvantage for battery life.
JP-A-7-97648, for example, proposes another measure for improving the electrode capacity, wherein a portion of the alloy is substituted by Mn, and the alloy melt is rapidly cooled and solidified to form a columnar crystal structure of a particular size. This method gives the alloy a fine crystal structure by the rapid cooling and reduced segregation of Mn, thereby improving the electrode capacity and cycle life to a certain degree. However, excess amount of Mn causes corrosion at the segregation to lower the cycle life, and no remarkable improvement is achieved in the high rate charge-discharge performance compared to that of the conventional alloy.
In an attempt to improve the high rate charge-discharge performance, there is proposed to plate the surface of a hydrogen storage alloy with nickel for utilizing the catalytic effect of nickel. However, repeated charging and discharging of the battery decrepitates the alloy to form fresh surfaces, thereby diminishing the effect of nickel plating.
It is conventionally believed to be essential for improving the corrosion resistance against the electrolyte and for improving the battery life, to add Co in an amount of not less than 4% by weight, usually about 10% by weight. However, addition of Co adversely affects the activity of the hydrogen storage alloy (easiness to desorb hydrogen), and adds greatly to the cost of the alloy. Therefore, substitution for Co addition is demanded.